all: add GEMINI.md #19075

Job	Run time
old	7m 44s
gvisor	4m 10s
race	6m 17s
race_dashboard	3m 9s
aux	3m 25s
build	3m 12s
dashboard	2m 19s
arch (presubmit_arch_darwin)	3m 54s
arch (presubmit_arch_windows)	3m 40s
arch (presubmit_arch_netbsd)	3m 34s
arch (presubmit_arch_linux)	7m 57s
arch (presubmit_arch_freebsd)	6m 32s
arch (presubmit_arch_openbsd)	3m 37s
arch (presubmit_arch_executor)	2m 59s
	1h 2m 29s

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